Microzooplankton herbivory in the Ross Sea, Antarctica

Microplankton abundances and phytoplankton mortality rates were determined at six stations during four cruises spanning three seasons in the Ross Sea polynya, Antarctica (early spring, Oct.-Nov. 1996; mid-late summer, Jan.-Fe b. 1997; fall, Apr. 1997; mid-late spring, Nov.-Dec. 1997). Rates of microz ooplankton herbivory were measured using a modified dilution technique, as well as by examining the rate of disappearance of phytoplankton (chlorophyl l) in samples incubated in the dark (i.e. grazing in the absence of phytopl ankton growth). Strong seasonal cycles of phytoplankton and microzooplankto n abundance were observed during the study. Microzooplankton abundance vari ed by more than three orders of magnitude during the four cruises, and was positively correlated with phytoplankton biomass over the entire data set. Nevertheless, microzooplankton grazing was insufficient to impact significa ntly phytoplankton standing stocks during most of the experiments performed in this perenially cold environment. Only thirteen out of a total of 51 ex periments yielded phytoplankton mortality rates that were significantly dif ferent from zero. The highest mortality rate observed in this study (0.26 d (-1)) was modest compared with maximal rates that have been observed in tem perate and tropical ecosystems. Results from twenty experiments examining t he rate of decrease of phytoplankton biomass during incubations in the dark agreed quite well with the results of the dilution experiments performed a t the same time. The range of mortality rates for the dark incubations was - 0.09-0.06 d(-1), and the average was essentially zero (- 0.01 d(-1)). Tha t is, chlorophyll concentration was virtually unchanged in samples incubate d in the dark for 3 d. A number of factors appeared to contribute to the ve ry low rates of microbial herbivory observed, including low water temperatu re, and the size and taxonomic composition of the phytoplankton assemblage. Based on our results we conclude that the seasonal, massive phytoplankton blooms observed in the Ross Sea are due, in part: to low rates of removal b y microbial herbivores. (C) 2000 Elsevier Science Ltd. AII rights reserved.